Writing+Chemical+Equations

//Adapted from **David A. Katz**// //Chemist, Educator, Science Communicator, and Consultant// //Department of Chemistry, Pima Community College//

A chemical equation is a chemist's shorthand expression for describing a chemical change. As an example, - consider what takes place when iron rusts. The equation for this change is:
 * I. The Meaning of a Chemical Equation**

Fe + O2 --> Fe2O3

In this expression, the symbols and formulas of the reacting substances, called the **reactants**, are written on the left side of the arrow and the **products** of the reactions are written on the right side. The arrow is read as "gives", "yields", or "forms" and the plus (+) sign is read as "and". When the plus (+) sign appears between the formulas for two reactants, it can be read as "reacts with". (The + sign does not imply mathematical addition.)

The equation, above, can be read as //iron reacts with oxygen to yield (or form) iron(III) oxide.//


 * II. Balancing a Chemical Equations**

As it is written, the equation indicates in a qualitative way what substances are consumed in the reaction and what new substances are formed. In order to have quantitative information about the reaction, the equation must be balanced so that it conforms to the //Law of Conservation of Matter//. That is, there must be the same number of atoms of each element on the right hand side of the equation as there are on the left hand side.

If the number of atoms of each element in the equation above are counted, it is observed that there are 1 atom of Fe and 2 atoms of O on the left side and 2 atoms Fe and 3 atoms of O on the right.

Fe + O2 --> Fe2O3

//Left side: Right side:// 1 atom Fe 2 atoms Fe 2 atoms O 3 atoms O

The balancing of the equation is accomplished by introducing the proper number or **coefficient** before each formula. To balance the number of O atoms, write a 3 in front of the O2 and a 2 in front of the Fe2O3:

Fe + 3 O2 --> 2 Fe2O3

The equation, above, now has 6 atoms of O on each side, but the Fe atoms are not balanced. Since there is 1 atom of Fe on the left and 4 atoms of Fe on the right, the Fe atoms can be balanced by writing a 4 in front of the Fe:

4 Fe + 3 O2 --> 2 Fe2O3

This equation is now balanced. It contains 4 atoms of Fe and 6 atoms of O on each side of the equation. The equation is interpreted to mean that 4 atoms of Fe will react with 3 molecules of O2 to form 2 molecules of Fe2O3.